Hearts of 78 necropsy patients with trasmural acute myocardial infarction were studied to correlate the mode of death, the interval between onset of infarction and death, and the presence or absence of coronary thrombus with the extent of the infarct. Infarct size was assessed quantitatively as a percent of total left ventricular mass. Death was caused by cardiogenic shock in 16 patients (21%), arrhythmia in 31 patients (40%), and cardiac rupture in 24 patients (13%). The mean interval between the onset of acute myocardial infarction and death was 12 plus/minus 13 days. Infarct size averaged 23 minus/plus 14% left ventricular mass. Patients who died in cardiogenic shock had the largest infarcts (37 plus/minus 11%) and those dying of cardiac rupture had the smallest infarcts (15 plus/minus 9%) and the shortest interval between onset of infarction and death (7 plus/minus 8 days). Coronary thrombi were present in 58 patients (74%). When present, thrombus was observed in h coronary artery which had supplied the infarct area and was superimposed on advanced atherosclerotic plaque but there was no apparent relation between the extent of luminal obstruction by thrombus and infarct size. The absence of coronary thrombus at necropsy was associated with either small infarcts or prolonged survival following infarction.